1. Field of the Invention
The present invention describes a method of Pyrolytic Heat Recovery Enhanced with Gasification (PHREG) for a wide variety of heterogeneous organic materials. The invention proper is limited to the design and operation of reactor and driver gas generation facilities, but an entire process built around the reactor is described in some detail to provide context and clarity of concept.
The process of this invention converts heterogeneous organic materials such as municipal refuse, coal, biomass, agricultural wastes, hazardous wastes, petroleum coke and oil shale, individually or as mixtures, into valuable high energy content gas. This gas can is suitable for making hydrogen, synthetic hydrocarbons, and other industrial chemical products, or for burning to recover fuel values. The process achieves a significant reduction in volume (approximately 20 fold) and weight (approximately 10 fold) of residual solid material. It combines high temperature pyrolysis of a heterogeneous solid organic feed materials and anaerobic gasification of the resulting pyrolyticly generated char material.
2. Description of Related Art
Mankind generates immense amounts of wastes as a result of daily living. Many of these wastes result directly from individual behavior, but many others result from agricultural and industrial activities that provide various goods that we buy and use. Many of these wastes are collected and treated or disposed of by municipal waste utilities; examples are sewage treatment and garbage pickup. Some of this waste is converted to useful products (e.g., aluminum cans and newsprint), but most eventually ends up buried in the ground in landfills. In the past, and in many places even today, the volumes buried in landfills are relatively small compared to the available space for them; but every US community with more than about a million citizens faces an uncertain future as the available landfill space is beginning to be exhausted. Some cities are shipping wastes great distances for disposal, and resistance to receiving these wastes is growing. In an extreme example, during the 1990s a barge load of wastes from New York City traveled from port to port for months looking for a suitable disposal site, eventually returning to New York.
Efforts in the past to recycle the energy values while reducing the volume of these wastes have been relatively unsuccessful. A survey of available technical literature and issued patents on processes for the pyrolytic (thermal) decomposition of organic matter in such wastes shows a large amount of information stems from projects funded by the United States Department of Energy. Such projects range in scope from outright incineration (probably 85-90% of the work) of municipal waste, coal, and biomass to true pyrolytic processes where no oxygen is brought into contact with the wastes during treatment. Most of this work was conceived and funded for study in the late 1970s and early 1980s in the aftermath of the Arab and Iranian oil crises (1).
Many of the resulting processes which began with efforts to use other organic material, mostly coal, though several projects using biomass and municipal solid waste have also been funded and even commercialized to some extent. Several of these plants have been started and successfully run for several years. Some of them are still operating, especially in areas of the world where indigenous oil and gas are rare but coal is plentiful.
Water gas production from the reaction of steam with coal or coke was the principal means of providing municipal heating and lighting gas for the first thirty years of this century. South Africa is an example, where the SASOL coal gasification complex has successfully supplied gasoline, chemicals and synthetic natural gas to that nation for over 40 years. Germany supplied much of its war needs in the 1940's from coal pyrolysis and gasification. (4) Dakota Gasification Co. operates large 300 MW high sulfur lignite to Synthetic Natural Gas (SNG) plant in Beulah, N. Dak. Texaco has demonstrated a gasification technology for nearly 30 years based on partial oxidation as well as steam-gas reforming. Most of these facilities take the produced “syngas” and convert it to various types of fuels and/or chemicals. This type of technology, using mostly natural gas for simplicity, is the primary way that hydrogen is made for thousands of refineries and fertilizer plants in the world today.
Most of the currently existing facilities and project proposals for reducing the volumes of various solid wastes center on technologies that can be best described as incineration. In processes based on these technologies, the wastes are combusted in the presence of excess air or oxygen, with some amount of heat recovered from the resulting flue gases. While the apparent simplicity of incineration technologies has much to recommend them, there is an important added complexity of trace noxious and hazardous compounds in the product gases. Such gases include oxides of sulfur (SOx) from the burning of sulfur-containing compounds in the feed. They also include nitrogen oxides (NOx) from the reaction of oxygen with nitrogen in the air during combustion. Other problem emissions include carcinogenic dioxins, which form during combustion of various waste materials. Other undesirable gases can also be found in incinerator flue gas. As a result of these problems, incinerator flue gases must be treated to meet various national and local air emission standards. Such treatment is expensive, and the high costs have strongly impeded the development of incineration as a way to recover energy values and reduce the volume of wastes sent for disposal by burial in landfills.
Incineration Technology
Incineration of MSW has been used for years. The Baltimore Refuse Energy Systems Company (BRESCO) has been providing disposal of up to 2,250 tons per day of municipal solid waste from Baltimore City, Baltimore County, and other areas in Maryland since 1985. Trash is incinerated in three parallel processing units, each with a furnace, a boiler, and an air pollution control system. This is moving grate system generating high-pressure steam, in excess of 500,000 pounds per hour at full capacity. The steam is used to generate up to 60 megawatts of electricity, up to 60 megawatts, and for district heating and cooling. BRESCO is capable of supplying up to 300,000 pounds of steam per hour to a district heating facility, which distributes the steam to buildings in downtown Baltimore. This waste-to-energy facility successfully reduces the volume of incoming waste by approximately 90%, and recovers ferrous and non-ferrous metals from the ash residue. Energy recovery economics are said to be unfavorable because of the presence of large volumes of moisture during humid periods of the year.
A more recent facility in Long Beach, Calif., called SSERF, processes 1300 tons/day of MSW and generates some 50 MW of power in an extremely rigid air quality management district. This facility has the advantage of a mostly arid climate, improved environmental engineering techniques, and local support subsidies.
Economics for many of these projects in the 1970s and early 1980s were based on the high value of the energy produced. At that time the potential shortage of U.S. landfill options was not a factor. Federal, state, and municipal governments and private investors funded these and other similar waste recovery schemes as alternative fuel plants.
Thermo-chemical Conversion Processes
The patent literature is rich with examples of processes that have been conceptualized and developed for non-incinerating thermal conversion of wastes and other organic solids to make mixtures of gas, liquid, and (usually) char products. Most of these gasifiers use a combination of chemical gasification of feed material and pyrolytic cracking of the feed material molecules. The chemistry of these processes is well established and verified.
Pyrolysis is commonly defined as thermal decomposition in an environment of less-than-stoichiometric oxygen; thus, partial oxidation reactors have been placed lumped under this category. The funded projects can be split into two subcategories—those that employ some form of organic combustion and those that do not. The former category of projects was mostly coal-based gasification projects, which used direct (though incomplete) combustion of feed material to generate the necessary reaction heat. The latter processes use externally derived heat, often from solar or nuclear generated sources. A study of such processes provides guides to materials of construction, performance, and perhaps some decomposition kinetic rate, and yield chemistry.
In addition, several of these processes heat the organic feed material only to the point of leaving a carbon-ash composite solid as a reactor product/waste. Some of the more advanced, and most technically viable, processes utilized this residual char material as fuel for burning (usually within the process but outside the reactor) to generate the required process reaction heat. An example of this was the Garrett Process developed by Occidental Petroleum, creating a gas, a foul-smelling oil with low BTU value, and a char (U.S. Pat. Nos. 4,153,514, 4,162,959, 4,166,786, to Garrett, Mallan, Durai-Swamy, etc.). The char was burned outside the pyrolysis reactor to generate the required heat, and the resulting hot char was recycled to heat the incoming feed. Other processes used pure oxygen or oxygen-enriched air to increase the temperature at the bottom of the reactor, since standard air combustion will realistically allows for only about a 1800 or 2000° F. maximum reactor temperature. Oxygen-enrichment also reduces the problems associated with having nitrogen in the reaction system, these primarily being NOx formation and reduction of final gas heating value.
Depending upon the composition, ash will melts into a viscous slag phase at temperatures of 2000 to 2700° F. Metals will melt from 1500° F. to as high as 3700° F. (tungsten), often necessitating oxygen-enriched combustion air in order to obtain the reaction temperatures needed to recover ash and metals as melts and properly gasify the carbonaceous and other volatile material in the char.
In most of these thermo-chemical processes, the volume of solid by product is 2 to 4 percent of that of incoming refuse, depending upon the amount of noncombustible materials in the mixed wastes. By contrast, a well-designed and efficiently-operated conventional incinerator produces a solid residue of 10% or more of the volume of refuse burned.